Titan coated in fluffy wet dust dunes?

Bored with smog? Tired of the tedium of dust? Then spice up your life by looking to Titan, Saturn's largest moon, where the surface may be covered with a thick layer of 'smust'.

Donald Hunten, at the University of Arizona's Lunar and Planetary Laboratory, proposes the existence of smust as the answer to the confusion about ethane on Titan, a moon known to have an atmosphere choked with carbon-heavy clouds.

This weather system has caused space scientists to scratch their heads for a number of years. First they thought oceans of ethane and methane were the explanation for the organic-stuffed atmosphere. When no oceans could be found, lakes were proposed (and then spotted). But still the amount of ethane found swimming in lakes on the surface is lower than researchers might predict from data of the moon's atmosphere.

Hunten used data from the Galileo mission to look at ethane distribution on Jupiter. He noticed that ethane was sequestered onto smog particles at low temperatures there conditions similar to those on Titan. "The application to Titan jumped out at me," he says.

Hunten thinks that ethane gets stuck on dust particles in Titan's thick atmospheric haze and stays stuck as it falls to the ground, unable to deposit itself in a lake or an ocean. The result? Based on data about the size of particles on Titan's surface, he calculates there must be a layer of fluffy ethane-soaked dust an average of 2.6 kilometres deep. He reports the theory in Nature.1

"I wouldn't say the idea was crazy," says Tetsuya Tokano, a Titan weather watcher from the University of Cologne, Germany, of Hunten's idea. "It seems to exist on Jupiter." But he remains sceptical about Hunten's proposal, which is not backed up by any convincing data for Titan, he says.

Dusty dunes

Hunten also suggests that the dunes seen on Titan are made of smust. Mark Leese, of the Open University, UK, and part of the team that worked on the Huygens probe, which landed on Titan in January 2005, likes this part of Hunten's proposals. "It would be a neat way of giving a solution to what the material is in the dunes," he says. But Hunten's suggestion that the smust is kilometres deep is "hard to believe" he adds.

John Zarnecki, principal investigator on the Huygens Surface Science Package at the Open University notes that probe pictures don't seem to back the theory. "If this is a global phenomenon, this stuff should be everywhere," he says. "I don't see it in the images." He would also like to know whether the aerosol collector and pyrolyser that collected data from Titan has any evidence of these bound ethane particles. "You'd expect some signature," he says.

"Smust might be part of the answer," says Leese. But Leese and Zarnecki expect to find more lakes on Titan that should make up a good proportion of any 'missing' ethane. On 9 October, the Cassini craft made its latest flyby of the moon and the data are due any day now although this probably won't close up all the mysteries.

To verify the smusty claims, lab recreations of Titan are needed. This is going to be difficult. "I don't see any way they could be carried out," says Hunten. The smog particles are very fluffy, he explains, and if they were produced in a lab they would instantly collide with the walls of the chamber and be modified or destroyed. "I wish my lab colleagues the best of luck, but I am not optimistic."